Cessna 560 N654CE accident near Aledo, Texas

Probable cause

The in-flight separation of the upper and lower right engine cowlings for reasons that could not be determined based on available evidence.

NTSB analysis

The air transport pilot of the twin-engine jet heard a "loud crashing noise" followed by a "turbulent air sound." The pilot reported everything checked out normal except the "turbulent air sound," which was exacerbated with rudder movement. The flight diverted and continued to a normal landing. A postflight inspection revealed the upper and lower cowlings for the No. 2 (right) engine were missing. A portion of the separated cowlings was found wrapped around the inboard section of the right horizontal stabilizer. Another portion of the separated cowlings impacted the area between the leading edge of the vertical stabilizer and the top of the fuselage, resulting in substantial damage. The remaining pieces of the separated cowlings have not been recovered.

On-scene examination revealed that receptacle 29, at the forward inboard side of the lower cowl door, was missing its stud, snap ring, and grommet and that receptacles 26 and 28 were missing their grommets and snap rings but had retained their studs. All other receptacles had retained their attachment hardware. Receptacle 29 was visually examined for indications of damage or wear and the locking features on the backside of the cup were compared against an exemplar cup. There were no apparent indications of damage or wear that could have contributed to the stud cross-pin pulling out or rotating out of the locked position. The fracture surfaces on the two cowl pieces were examined for indications of fatigue, but none were found.

The most likely cause of the cowl door departure is that either the stud at receptacle 29 was left unlocked when the cowl door was attached, or an oversized stud was used at receptacle 29 that was able to rotate from the locked to unlocked position. Without the stud to examine and additional on-wing testing, it cannot be determined whether the stud at that location was a longer grip-length stud or if spontaneous unlocking of a longer grip-length stud is an actual possibility.

The reason for the separation of the cowlings could not be determined because the stud in question was not recovered.

Text from the NTSB. Open the NTSB docket for CEN20LA082 for the full record.